1. Field of the Invention
The present invention relates to a thermally-assisted magnetic head that records information while heating a magnetic recording medium to reduce coercive force of the magnetic recording medium.
2. Description of the Related Art
In recent years, regarding magnetic recording devices such as a magnetic disk device, etc., improvements have been demanded in the performance of a magnetic head and a magnetic recording medium in conjunction with high recording density. As the magnetic head, a composite-type magnetic head is widely utilized in which a reproducing head including a magneto resistive effect element (MR element) for reading and a magnetic recording head including an inductive-type electromagnetic transducer (a magnetic recording element) for writing are laminated on a substrate. In the magnetic disk device, the magnetic head flies slightly above a surface of a magnetic recording medium.
The magnetic recording medium is a discontinuous medium on which magnetic microparticles gather. Each of the magnetic microparticles has a single magnetic domain structure. One recording bit in the magnetic recording medium is configured with a plurality of the magnetic microparticles. In order to enhance the recording density, asperities on a boundary between adjacent recording bits must be reduced in size. For this, the magnetic microparticles should be reduced in size. However, reducing the magnetic microparticles in size leads to a decrease in the volume of the magnetic microparticles, resulting in a decrease in thermal stability of magnetization in the magnetic microparticles. In order to solve this problem, increasing the anisotropy energy of the magnetic microparticles is effective. However, when the anisotropy energy of the magnetic microparticles is increased, the coercive force of the magnetic recording medium is also increased. As a result, it becomes difficult to record information utilizing a conventional magnetic recording head. Conventional magnetic recording heads have such a drawback, and this is a large obstacle to achieve an increase in the recording density.
As a method to solve this problem, a so-called thermally-assisted magnetic recording method has been proposed. In this method, a magnetic recording medium with large coercive force is utilized, and heat as well as the magnetic field is applied to a portion, to which information is recorded, of the magnetic recording medium when recording the information. Therefore, the information is recorded under a state where the temperature is increased and the coercive force is decreased in the information recording portion.
For thermally-assisted magnetic recording, a method in which a laser light source is utilized to heat the magnetic recording medium is common. Two types of this method include: a method of heating the magnetic recording medium by guiding laser light to a recording portion via a waveguide, etc. (a direct heating); and a method of heating the magnetic recording medium by converting laser light to near-field light (a near-field light heating). Near-field light is, so to say, a type of electromagnetic field that is formed around substances. Ordinary light cannot be tapered to a smaller region than its wavelength due to diffraction limitations. However, when light having an identical wavelength is irradiated onto a microstructure, near-field light depending on the scale of the microstructure is generated, enabling the light to be tapered to a minimal region being approximately tens of nm in size. Since the thermally-assisted recording targets a recording density region that requires selective heating only to the minimal region being approximately tens of nm, the near-field light heating is preferred.
U.S. Patent Application Publication No. 2008/0205202 discloses a configuration in which a near-field-generator is disposed in a front part of a core of a waveguide through which light from a laser diode (LD) propagates.
As a specific method of generating the near-field light, a method utilizing a so-called plasmon antenna, which is a metal referred to as a near-field light probe that generates near-field light from light-excited plasmon, is common.
In the plasmon antenna, the near-field light is generated by directly irradiating light; however, conversion efficiency of converting irradiated light into the near-field light is low with this method. Most of the energy of the light irradiated on the plasmon antenna reflects off the surface of the plasmon antenna or is converted into thermal energy. The size of the plasmon antenna is set to the wavelength of the light or less, so that the volume of the plasmon antenna is small. Accordingly, the temperature increase in the plasmon antenna resulting from the light energy being converted into the thermal energy is significantly large.
The temperature increase causes volume expansion of the plasmon antenna, and the plasmon antenna protrudes from an air bearing surface (ABS) that is a surface facing the magnetic recording medium. Then, the distance between an edge part of the MR element on the ABS and the magnetic recording medium increases, causing a problem that servo signals recorded on the magnetic recording medium cannot be read during the recording process. Moreover, when the heat generation is large, the plasmon antenna may melt.
Currently, a technology is proposed in which light is not directly irradiated onto the plasmon antenna. For example, U.S. Pat. No. 7,330,404 discloses a technology in which light propagating through a waveguide such as an optical fiber, etc. is not directly irradiated onto the plasmon antenna; however, the light is coupled with a plasmon generator in a surface plasmon mode via a buffer portion to excite a surface plasmon in the plasmon generator. The plasmon generator includes a near-field-generator that is positioned on the ABS and that generates the near-field light. At the interface between the waveguide and the buffer portion, the light propagating through the waveguide completely reflects off, and light, which is referred to as evanescent light, is simultaneously generated that penetrates into the buffer portion. The evanescent light and a collective oscillation of charges in the plasmon generator are coupled, and the surface plasmon is then excited in the plasmon generator. The excited surface plasmon propagates to the near-field-generator along the plasmon generator, and then generates near-field light in the near-field-generator. According to this technology, since the light propagating through the waveguide is not directly irradiated to the plasmon generator, excessive temperature increase in the plasmon generator is suppressed.
U.S. Patent Application Publication No. 2010/0103553 discloses a configuration in which a propagation edge is disposed in a plasmon generator that couples to light in a surface plasmon mode. The propagation edge that is an extremely narrow region is for propagating a surface plasmon generated in a plasmon generator to a near-field-generator positioned on an ABS.
In thermally-assisted magnetic recording that records while heating predefined portions of the magnetic recording medium, a temperature increase in the thermally-assisted magnetic head itself as well as the magnetic recording medium cannot be prevented. A loss generated when a surface plasmon propagates through a propagation edge of the plasmon generator is a major factor in the temperature increase of the thermally-assisted magnetic head. Specifically, when the plasmon generator is formed in protuberant shape toward a core on the ABS as disclosed in U.S. Patent Publication 2010/0103553, migration due to temperature increase is more likely to occur at a tip part in the protuberant shape toward the core. The migration in the plasmon generator may lead to lower output of the thermally-assisted magnetic head.
Note, JP Laid-Open Publication 2002-304708 discloses a configuration of a magnetic head without thermal assistance in which an ABS-side surface of a pole of a magnetic recording element is covered with a high corrosion-resistant magnetic film. Since the magnetic head is not made for thermally-assisted magnetic recording, the magnetic head does not include a plasmon generator, and also migration due to the temperature increase in the plasmon generator is not noticed.
The invention disclosed in U.S. Pat. No. 7,529,158 has a configuration in which a plasmon generator is configured as a composite material made from two materials. Specifically, FIG. 24A discloses a configuration in which the vicinity of a vertex of the plasmon generator on an ABS side is formed of Pt, Pd, Rh, Ir, Ti, Cr, Co, Si or SiN having high hardness. The configuration allows the mechanical durability to improve with the materials with high hardness; however, the configuration is not for preventing migration resulting from the temperature increase.